1. Field of the Invention
The present invention relates to a process for the hydraulic fracturing of subterranean formations surrounding oil wells and gas wells by means of injection of a fracturing fluid into the well, wherein the fracturing fluid comprises a solution of a nonpolar organic liquid or oil, an interpolymer complex and optionally a polar cosolvent, propping agent, and soluble or insoluble additives.
2. Description of the Prior Art
Hydraulic fracturing has been widely used for stimulating the production of crude oil and natural gas from wells completed in low permeability reservoirs. The methods employed normally require the injection of a fracturing fluid containing a suspended propping agent into a well at a rate sufficient to open a fracture in the exposed formation. Continued pumping of fluid into the well at a high rate extends the fracture and leads to the buildup of a bed of propping agent particles between the fracture walls. These particles prevent complete closure of the fracture as the fluid subsequently leaks off into the adjacent formation and result in a permeable channel extending from the wellbore into the formation. The conductivity of this channel depends upon the fracture dimensions, the size of the propping agent particles, the particle spacing, and the confining pressures. Studies of conventional fracturing operations indicate that fracture widths seldom exceed about one-fourth inch and that conductivities in excess of about 250,000 millidarcy inches are rarely obtained. The average width and conductivity are considerably lower than these values.
With the advent of declining reserves, the drilling and stimulation of higher temperature wells in increasing the drilling and completion of tight dry gas and water sensitive formation is also on the rise. The industry is relying to a greater extent on hydrocarbon fluids to drill and complete these wells which cannot be treated with the normal water-based fluids, therefore, there has been shown a substantial need for hydrocarbon based viscosifiers which exhibit good performance at high temperature.
The incentives for developing fractures with conductivities sufficient to permit the application of fracturing to high permeability reservoirs are substantial. The low permeability formations in which conventional methods are used generally produce at low rates and hence total production remains low even though an improvement of several fold is obtained. In reservoirs of higher permeability, the initial production rates are normally much higher and hence a successful fracturing operation may produce a much greater improvement in terms of incremental barrels of oil per day. This is true even though the percentage improvement may be somewhat smaller than in a reservoir of lower permeability. Efforts to extend fracturing operations into undamaged reservoirs with permeabilities substantially in excess of about 15 to 20 millidarcies have in the past been largely unsuccessful.
A desirable formulation for hydraulic fracturing fluids for these difficult fracturing situations would be a fluid which possesses adequate viscosity to support the propping agent and to aid in propagating the fracture. Such a formulation would be an organic liquid containing at least one polymeric viscosifier.
Since the beginning of recorded oil well production, organic based viscosifiers have played an important role in hydraulic fracturing fluids. Some of these viscosifiers have been either metal soaps of fatty acids, or metal soaps of partially esterified phosphates. Both of these impart viscosity to hydrocarbons, but the metal soaps of fatty acids have inherent thermal thinning properties which give them limited utility at higher temperatures. The metal soaps of partially esterified phosphates have the disadvantages of being extremely pH sensitive along with being thermally thinning.
Therefore a viscosifier that has the advantage of maintaining viscosity at high temperatures and/or is not susceptible to variations in pH would represent an advancement of the prior art.
The instant invention differs from a number of applications, Ser. Nos. 223,482, now U.S. Pat. No. 4,361,658; 136,837, now U.S. Pat. No. 4,322,329; and 106,027, now U.S. Pat. No. 4,282,830, filed by Robert Lundberg et al, one of the instant inventors. These previously filed applications were directed to the gelling of the organic liquid by a water insoluble, neutralized sulfonated polymer whereas the instant invention is directed to fracturing fluids formed from nonpolar organic liquid and an interpolymer complex.
In a U.S. Ser. No. 547,955, filed Nov. 2, 1983, now abandoned, two polymers are mixed to produce an interpolymer complex which at relatively low concentration forms a three-dimensional network with a gel-like behavior.
In U.S. Ser. No. 547,955 the interpolymer complexes in hydrocarbon solutions are obtained by mixing two polymers which are strongly associating, with each other. One polymer will contan metal neutralized sulfonate groups along or pendant to its backbone, and the other polymer will contain basic amine groups. This interaction between metal neutralized and basic amine groups leads to a dynamic network formation, if each chain contains interacting groups in multiple locations.
In this application the use of this multi-polymer complex is described as a hydrocarbon viscosifier when sufficient hydrocarbon character is present in the polymers to allow solubility in hydrocarbon, particularly crude and semi-refined hydrocarbon solvents. The concentrations of multi-polymer complexes are described and can be varied to obtain a range of rheological properties. Furthermore, this invention discloses that by variations of sulfonation level on the sulfonated polymer, metal cation on the sulfonated polymer, level of amine in the amine containing polymer, and other parameters a range of rheological properties can be obtained and that finally, these multi-polymer complexes are useful in viscosifying hydrocarbon solutions for hydraulic fracturing.